Chemistry 9701/33 October/November 2019: question paper and mark scheme

Mark scheme

1(a) I All the following data recorded 1 • two burette readings and titre for the rough titration • initial and final burette readings for two (or more) accurate titrations II Titre values shown, for accurate titrations, and appropriate headings and units in the accurate titration table 1 • initial / start and (burette) reading / volume • final / end and (burette) reading / volume • titre or volume / FB 4 and used / added • unit: / cm3 or (cm3) or in cm3 (for each heading) or cm3 unit given for each volume recorded III All accurate burette readings are recorded to the nearest .05 cm3. 1 IV The final accurate titre recorded is within 0.10 cm3 of any other accurate titre. 1 Award V if δ ⩽ 0.80 (cm3) (Where δ is difference to the supervisor’s value) 1 Award VI if δ ⩽ 0.50 (cm3) 1 Award VII if δ ⩽ 0.30 (cm3) 1 1(b) Correctly calculates mean titre from two (or more) accurate titres where the total spread is ⩽ 0.20 cm3. 1 AND Answer is given to 2 d.p. AND Working must be shown or ticks must be put next to the two (or more) accurate titres selected. 1(c)(i) Significant figures 1 All answers in (ii)–(iv) are expressed to 3 or 4 sig. fig. 1(c)(ii) Correctly calculates no of moles of NaOH used. 1 No. of moles NaOH = 6.00/40 × answer (b)/1000 1(c)(iii) NaOH(aq) + HCl(aq) → NaCl(aq) + H2O(l) 1 or H+(aq) + OH-(aq) → H2O(l) AND No. of moles of HCl = same as in moles of NaOH in (ii) 1(c)(iv) Correct expression shown (for the first step) 1 (Two steps) Concentration HCl in FA 3 = ans (c)(iii) × 1000 / 25 Allow ans (c)(iii) × 40 (Concentration of FA 2 = Concentration of FA 3 × 25) OR moles of FA 3 in 250 cm3 = ans (c)(iii) × 250 / 25 Allow ans (c)(iii) × 10 (moles of FA 2) in 1 dm3 = ans above × 1000 / 10) Answer = (c)(iii) × 1000 1

Mark scheme

2(a) I Suitable tables/lists with values entered in the space provided. 1 • Three clearly labelled masses (do not allow ‘weight’) with correct units • Three unambiguous labelled temperatures, with units II Readings and subtraction in 2(a) and 2(d) 1 • All four measured temperatures recorded to .0 or .5 °C • Both temperature changes correctly subtracted. • All masses in (a) and (d) recorded to one or more d.p. (balance readings must be consistent d.p. within each experiment) • Both masses correctly subtracted. Accuracy marks 2 Round all thermometer readings to 0.5 °C. Check the candidate’s and supervisor’s subtractions and compare the candidate’s [corrected] temperature rise with the supervisor’s. The difference between candidate and supervisor is δ. See table below for marks available. Sup ∆Tmax > 40.0 °C 30.5–40.0 °C 20.5–30.0 °C 10.5–20.0 °C 5.5–10.0 °C < 5.5 °C 1 mark δ ⩽ 5.0 °C δ ⩽ 4.0 °C δ ⩽ 3.0 °C δ ⩽ 2.0 °C δ ⩽ 1.0 °C δ ⩽ 0.5 °C δ ⩽ 2.5 °C δ ⩽ 2.0 °C δ ⩽ 1.5 °C δ ⩽ 1.0 °C δ ⩽ 0.5 °C not available 2 marks 2(b)(i) Correctly calculates heat change 1 Heat change = 40 × 4.2 × temp rise AND Answer is correct to 2–4 s.f. 2(b)(ii) Correctly calculates number of moles of MgO 1 No. of moles of MgO used = mass used/40.3 AND Answer is correct to 2–4 s.f. 2(b)(iii) Correctly uses (i)/(ii) and negative sign shown 1 Enthalpy change = − (b)(i)/(b)(ii) × 1/1000 AND Answer is given to 2–4 s.f. 2(c) Correct expression(s) shown in ‘excess’ calculation 1 Candidate must compare the number of moles of MgO and HCl used and correctly use the mole ratio of 1:2. Example of working No. of moles of HCl used = 0.04 × ans 1(c)(iv) (or n = 0.04 × 3.75 = 0.15) Maximum no. moles MgO that can be used = 0.5 × n Note – there are other valid ways of doing this calculation. 2(d)(i) Readings written in space provided 1 • Two thermometer readings are recorded both above 10 °C • Two masses are recorded giving mass of MgCO3 between 2–5 g. Accuracy mark 1 Check temperature subtractions of candidate and of supervisor. Compare the [corrected] candidate’s temperature rise with the supervisor’s. The difference is δ. • If δ is less than or equal to 2.0 °C, award the mark 2(d)(ii) Observations (two required) 1 Fizzing / bubbles / effervescence AND either solid dissolves / disappears / colourless solution formed or reaction is brisk / rapid / vigorous / violent 2(d)(iii) Correct expression(s) for enthalpy change, with negative sign 1 Enthalpy change = −40 × 4.2 × temp rise × 84.3/ mass MgCO3 ×1/1000 2(e) Enthalpy change correctly calculated with correct sign 1 Answer = (b)(iii) – (d)(iii) and to minimum 2 s.f. unless answer is an integer. If default values are used, the answer must be −44.4 kJ mol−1. 2(f) Plot a cooling curve (after the maximum temperature reached) 1 OR plot a graph to get better value of ∆T OR use increased masses of FA 4 and / or FA 5 / mass of solid (added) OR heat the acid before adding solid / FA 4 and / or FA 5 Ignore answers related to the apparatus used. 2(g) Correct single reading error (see below) and MgO has the greater % error 1 (Allow MgCO3 if this was the smaller mass.) If candidate used a 1 d.p. balance for this solid, error is 0.05 or 0.1 g If candidate used a 2 d.p. balance for this solid, error is 0.005 or 0.01 g If candidate used a 3 d.p. balance for this solid, error is 0.0005 or 0.001 g Correct calculation of % error for MgO 1 % error = 2 × (single error/mass of MgO used) × 100 FA 6 = Zn; FA 7 = NaNO3; FA 9 = Na2SO3

Mark scheme

3(a) Observation 1 Bubbling / fizzing / effervescence (Ignore ‘gas formed’) Test for hydrogen during reaction 1 (Gas) pops with a lighted splint / burns with a pop (allow explodes) Deduction 1 FA 6 is a metal and gives hydrogen (with an acid) 3(b)(i) Heating FA 7 – observations 2 • FA 7) melts / (partially) dissolves / becomes liquid / becomes a solution • (liquid is) yellow or pale brown • Fizzing / bubbling / effervescence occurs • (Gas / oxygen) re-lights a glowing splint • Gas turns (blue) litmus red • After standing / cooling white / off-white / cream / paler solid (formed). Award 1 mark for two correct observations from the list, award 2 marks for four or more correct observations. MAX. 2 marks 3(b)(ii) Observations in the table (see below) 4 Expected observations are in the table below. Award 1 mark for every two correct observations (*) Reject no observation the first time seen. Reject a dash every time. test observation with FA 8 observation with FA 9 To a 1 cm depth in a test-tube, No (visible) reaction / no change or from purple to colourless add a few drops of acidified solution (in test tube) becomes pink / or KMnO4 / MnO4– potassium manganate(VII). purple / solution / KMnO4 / MnO4– stays decolourised purple * To a 1 cm depth in a test-tube, no reaction / no change / no precipitate / white precipitate (formed) * add a few drops of aqueous solution remains colourless * barium chloride or aqueous barium nitrate. To a 1 cm depth in a boiling tube, no reaction / no change / no precipitate * no reaction / no change / add an equal volume of aqueous no precipitate * sodium hydroxide. Warm carefully, then add aluminium foil. gas / ammonia turns (moist red) litmus fizzing / bubbling / effervescence blue * or gas / H2 pops with a lighted splint * 3(b)(iii) Both anions correct 1 • FA 8 = nitrate / NO3− • FA 9 = sulphite / SO32− 3(b)(iv) One relevant ionic equation with state symbols 1 • Ba2+(aq) + SO32−(aq) → BaSO3(s) • Ba2+(aq) + SO42−(aq) → BaSO4(s) • 2MnO4–(aq) + 6H+(aq) + 5SO32–(aq) → 5SO42–(aq) + 2Mn2+(aq) + 3H2O(l) • 3NO3–(aq) + 27H+(aq) + 3Al(s) → 3NH3(g) + 9H2O(l) + 3Al 3+(aq)